How Water Works
Illustrated processes, equ ipment, and technology
Lime Softening Removes Hardness-Causing Minerals
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N
atural water contains dissolved minerals, some of
which impart a quality known as “hardness.” Hard
water is caused by the presence of calcium and
magnesium, and lime softening removes these hardnesscausing minerals. Because of the special facilities required
and the complexity of the process, it’s applicable mainly to
medium-sized or large water systems where all treatment
can be accomplished at a central location to minimize cost.
4. A rapid-mix basin is used to thoroughly mix lime, soda
ash, and other chemicals with water and to maintain
mixing long enough for the chemicals to dissolve—
especially critical for lime, because it dissolves slowly.
5. Solids-contact clarifiers are often used to perform
coagulation–flocculation, sedimentation, sludge
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collection, and sludge recirculation. A solids-contact
basin saves on construction costs by handling many
softening processes in one component.
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6. F ilters used in a lime–soda ash softening plant are
identical to those used in a conventional rapid sand or
multimedia filter plant.
7. A recarbonation basin adds carbon dioxide, CO2, to
softened water to stabilize the water before it enters the
distribution system and to adjust the pH when two-stage
softening is practiced.
ILLUSTRATION: RON KNOWLTON,
engineering drawings: carollo engineers
3. Conventional dry feeders and solution feeders are used
to dispense coagulants, such as alum, and coagulant
and settling aids, such as organic polymers, if necessary.
Dust control equipment should always be used while
working with dry powdered chemicals.
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2. Large quantities of chemicals are used in the water
softening process. Lime can be purchased in two forms:
calcium hydroxide, Ca(OH)2, which is called hydrated lime
or slaked lime, and calcium oxide, CaO, which is called
quicklime or unslaked lime. Lime and sodium carbonate,
Na2CO3, also known as soda ash, are necessary when
there is a nominal amount of magnesium hardness in
the water. Sodium hydroxide, also known as caustic
soda, NaOH, can be used in place of lime or soda ash for
softening, but it’s generally more expensive.
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1a and 1b. Lime softening is used to treat either surface
water (1a) or groundwater (1b) sources.
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8. Sludge-dewatering devices such as vacuum filters reduce the
amount of water in the sludge, thereby making it easier to
handle. Most plants dispose of sludge by trucking it to a land
disposal site.
9. Chlorine is added for disinfection.
10. Finished water contact basins ensure contact time is
allotted for adequate disinfection.
1b
11. A covered clearwell stores water before it enters the
distribution system.
12. Pumps send safe, softened water throughout the community.
Editor’s Note: For more information on the pervasive problems associated with
hard water, see “Public Notice,” Opflow, May 2007, page 8.
Some illustration elements exaggerated for emphasis.
18 Opflow July 2007
2007 © American Water Works Association
www.awwa.org/communications/opflow
www.awwa.org/communications/opflow 2007 © American Water Works Association
July 2007 Opflow 19